1. Field of The Invention
This invention relates to a conductive polymer composition which comprises a conjugated double bond-bearing polymer doped with anions derived from anionic surface active agents and inorganic transition metal salts. The invention also relates to a process for preparing the conductive polymer composition and a process for forming the composition in the form of a film.
2. Description of the Prior Art
In general, conjugated double bond-bearing conductive polymers, such as polyaniline, polypyrrole and polythiophene, have been prepared through chemical oxidation polymerization or electrolytic polymerization.
When using electrolytic polymerization, the resultant conductive polymer is formed on an electrode in the form of a film, and is thus difficult to prepare the polymer in large amounts. In contrast, the polymerization through chemical oxidation has no limitation with respect to the amount. That is to say conductive polymers can be obtained in large amounts relatively easily by reaction between a polymerizable monomer and an appropriate oxidant.
However, the conductive polymer obtained by the chemical oxidation polymerization is in the form of a powder or particles. This makes it difficult to uniformly cover solid surfaces with the polymer. Hence, attempts have been made to form a film by mixing the polymer with various types of binders or by rendering the polymer soluble in solvents by introduction of an appropriate substituent thereinto.
Usually, the chemical oxidation polymerization velocity depends greatly on the type of oxidant. If ammonium persulfate or hydrogen peroxide which can increase the polymerization velocity is employed, the resultant polymer is not high in electric conductivity and/or is poor in heat resistance. The reason for this is that such an oxidant as mentioned above is so highly active that undesirable side reactions frequently take place. This may result in the formation of polymers whose structural regularity is not high, or once formed polymer may be degraded owing to the attack with the oxidant.
In order to solve the above problems, oxidants which contain transition metal ions such as trivalent iron ions are frequently used. Although, the resultant polymer has a relatively great electric conductivity and good thermal stability, the polymerization velocity is so small that it takes a long time before completion of the polymerization. Accordingly, the resultant conjugated double bond-bearing polymer is resident for a long time in the reaction system. Under these conditions, the polymer is most liable to be attacked with the protons generated during the course of the polymerization reaction, coupled with the problem that the electric conductivity eventually lowers.
The conjugated double bond-bearing conductive polymers have a dopant of anions or cations taken in as a part thereof, with which inherent electric, optical and chemical characteristics develop. The thermal stability of the polymer tends to depend greatly on the type of dopant. The anion doping in the conjugated double bond-bearing polymer is now described in more detail.
In general, inorganic acid ions are so small in ion size that they are likely to diffuse. Especially, under high temperature and high humidity conditions, the inorganic acid ions taken in a polymer matrix are relatively readily de-doped from the polymer matrix, making it difficult to provide a conductive polymer which has good heat and humidity resistances.
In order that polyvalent anions, such as sulfate ions, are permitted to incorporate in a polymer matrix as a dopant, it is necessary that two bonding sites which are positioned closely to each other, are provided within a polymer chain or between adjacent polymer chains. In addition, where the polyvalent ions are incorporated in the matrix as a dopant, they are extremely sterically crowded owing to the small ion size. This results in the polymer structure being strained, thus leading to lower electric conductivity than expected.
On the other hand, when organic anions having a bulky structure are used, little or no strain is not exerted on the polymer structure. Such bulky organic anions are not prone to diffusion on the application of heat. Thus, the resultant conjugated double bond-bearing polymers have good electric characteristics and good heat and moisture resistances.
In fact, when anthraquinonesulfonate ions or alkylnaphthalenesulfonate ions are doped, the resultant polypyrrole has good heat and moisture resistances as set out in Japanese Laid-open Patent Publication Nos. 3-93214 and 2-130906. In these publications, the doped polypyrrole is prepared by electrolytic polymerization. In the electrolytic polymerization, transition metal salts are usually used. However, the salts containing such relatively large molecule anions are low in solubility in water. Accordingly, it has been difficult to directly incorporate the anion into a conjugated double bond-bearing polymer through chemical oxidation polymerization in an aqueous medium.
The conductive polymers obtained by chemical oxidation polymerization are ordinarily in the form of powder or particles. Since the polymer structure is formed of conjugated double bonds, it is, in most cases, difficult to process the powder or particles by thermal melting or dispersion in solvents.
For the purpose of film formation, attempts have been made wherein the conductive polymer powder is mixed with resin binders, or appropriate substituents are introduced into the main chain of the polymer to render the polymer soluble in solvents.
However, the mixing with binder resins or the introduction of substituents into the main chain of the polymer has the following problems.
In the former case, it is inevitable that the electric conductivity lowers owing to the dilution of the conductive polymer. In the latter case where the polymer is rendered soluble in solvents, limitation is placed on the selection of a substituent owing to the difficulty in balancing the solubility to be imparted and the inherent characteristics, e.g. electric conductivity and environmental stability, of the conductive polymer. Additionally, the preparation of a polymerizable monomer having an appropriate substituent which is effective in rendering the resultant polymer soluble in solvents is difficult. This eventually leads to high costs of the conductive polymer.